Cytolytic mouse T cells were successfully retargeted with bispecific antibodies, so that they lysed, in vitro, syngeneic breast cancer cells induced by the mammary tumor virus. The bispecific antibody reacted with a triggering site on the surface of T cells - a CD3 component within the T-cell receptor complex,and with a virus-related antigen expressed on the surface of the tumor cells. Two requirements for the antitumor activity were that 1) the T cells had to be preactivated, and 2) the antibodies comprising the bispecific reagent had to be crosslinked and not simply mixed together. We preactivated the T cells by culturing splenocytes from normal donors with irradiated allogeneic cells, bacterial lipopolysaccharide, and recombinant IL-2. Preactivated mouse spleen cells also blocked the growth of syngeneic breast cancer cells in tumor neutralization (Winn) assays in vivo, if retargeted against the tumor cells with the bispecific antibodies.